Photoengraving resist



United States Patent 3,348,944 PHOTOENGRAVING RESIST Michael Michalchik,Bethpage, N.Y., assignor to Fairchild Camera & Instrument Corporation,Syosset, N.Y., a corporation of New York No Drawing. Filed July 17,1963, Ser. No. 295,798 '7 Claims. (Cl. 96-13) ABSTRACT OF THE DHSCLOSUREThis disclosure describes the preparation and use of a recording elementin which photoconductive zinc oxide is dispersed in an insulating filmcomposed of an hydroxylated silicone resin and sufiicient alkoxy phenylsilane or siloxane so that the alkoxy content of the filrn is from about0.3% to about 2% by weight.

This invention relates to improvements in the production of reliefimages. More particularly, it relates to improvements in the productionof relief images by a process which includes the production of a solventresistant polymeric image. The products thus obtained are useful formany purposes.

There are a number of commercial operations in which it is necessary orconvenient to prepare a relief image which accurately reproduces theoriginal subject. Photoengraving is one such operation, lithography isanother. Still another is the production of printed circuits forelectrieal devices. For convenience and ease of understanding, I willdescribe my invention as it applies to photoengraving, but it should beunderstood that the invention is also applicable to other processeswhere a relief image is necessary or useful.

Recently an electrostatic printing procedure has been developed whichpermits the production of a relief image by a novel process. In thisprocedure as applied to photoengraving, a recording element is firstproduced by coating a plate of zinc, magnesium or other etachable metalwith a coating composition comprising an electrical insulating film inwhich a photoconductive material such as zinc oxide is suspended.Photoconductive zinc oxide generally has a surface conductivity of atleast about 1O- ohm- /square/watt/cm. when exposed to a wave length of3900 A. In most operations, the ratio of zinc oxide to film varies fromabout 1:1 to about 12:1.

The coating is then made sensitive to light by substantially coveringthe surface with an electrostatic charge in the dark. One convenientmethod of charging is exposure of the surface to be charged to a coronadischarge produced by connecting one or more fine Wires to a directcircuit source, e.g. a direct circuit source of 3-10 kilovolts negative.The atmosphere surrounding the wires is ionized and the ion flowestablishes the charge density on the surface. This charge which isusually from 300 to 600 volts, decays slowly in the dark over a periodof minutes or hours depending upon the nature of the coating.

The recording element now sensitive to light is exposed by any of theconventional photographic processes and the electrostatic charge isseelctively leaked from the surface. The degree of charge decay on aparticular area of the charged surface of the recording element isdirectly proportional to the amount of light to Which the area isexposed. If light is projected onto the charged surface through aphotoengraving negative, the largest amount of charge decay will be inthe area of the recording element corresponding to the most transparentareas of the film. Charge decay in other areas will be correspondinglyless depending upon the transparency of the 3,343,944 Patented Oct. 24,1967 corresponding areas of the film. There is thus produced a latentelectrostatic image on the surface of the recording element whichaccurately reproduces the negative image on the film.

It is, of course, possible to effect selective charge decay by otherprocedures. For example, a contact print or a drawing may be focusedonto the charged surface of the recording element with the aid of anoptical system using light from one or more incandescent lamps.

The insulating film in which the photoconductive material is suspendedis a polymeric substance preferably a silicone resin which is capable offurther polymerization to produce a polymer of high molecular weight.

In the next step of the process, the latent electrostatic image isconverted to a polymeric image by controlled polymerization of the areasof the insulating film which have retained a charge. This isaccomplished by depositing a polymerization catalyst such as finelydivided aluminum octoate or resinate on the areas of the recordingelement bearing the electrostatic image. Deposition may be effected forexample by immmersing the recording element in a liquid carrier in whichthe catalyst is suspended. The liquid is charcterized by a high electricresistivity and is usually an aliphatic hydrocarbon or a mixture ofalpihatic hydrocarbons including heptane and homologues thereof. Thecatalyst particles, by a process which is not completely understood butis probably a triboelectric phenomenon, acquire a positive chargerelative to the surface of the recording element and are, thereforeattracted to the electrostatic image and de posited on it.

The recording element with the catalyst selectively deposited on itssurface is removed from the developer bath, i.e. the carrier liquid withthe suspended catalyst and rinsed. The purpose of the rinsing is toremove any catalyst particles which have been deposited on the surfaceof the recording element in areas other than those bearing theelectrostatic image. These randomly disposed catalyst particlesapparently deposit as a result of the relatively small electricpotential which remains in background areas of the insulating film afterproduction of the electrostatic image. Unless they are removed, theywill catalyze extraneous, undesirable polymerization in the backgroundareas. For some applications where high quality reproduction is notessential, this step may be omitted.

After rinsing, the recording element is placed in a curing oven whereheat polymerization of the catalyzed areas takes place with the resultthat a polymeric image is produced corresponding to the originalelectrostatic image. The polymeric image comprises areas of highmolecular Weight solvent resistant polymers surrounded by backgroundareas of low molecular weight solvent sensitive polymers. In the usualcase, the image area is resistant to a mixture of aromatic and aliphaticsolvents such as a 50:50 combination of heptane or kerosene and xyleneor toluene, but the background area is not resistant to saidcombination. The background area may, therefore, be selectivelydissolved to expose the metal surface which may be etched according tostandard procedures after cleaning or descumming to produce aphotoengraving resist.

The process described above is subject to deficiencies. One of the mostimportant deficiencies is the production of photoengraving resists inwhich the boundaries of the relief image are not sharply defined. Theresult is that reproductions which are produced using the resist areblurred and otherwise unsatisfactory for commercial use. Anotherimportant deficiency is the inability to achieve reproducible results inordinary industrial operations without tedious and extensive qualitycontrol procedures. Still another is the expense of the rinsing liquids.I have found that the basic cause of these deficiencies is theinsulating film hertofore employed in the production of recordingelements. For example, with the usual films, there is an erraticdiffusion of the catalyst with the result that the pattern ofpolymerization spreads beyond the boundaries of the originalelectrostatic image with the result that the boundaries of the polymericimage ultimately produced is not sharp and well defined.

The insulating films are usually produced by spraying the insulatingbase surface with solutions of the partially polymerized insulating filmmaterial with zinc oxide or other photoconductive substance suspendedtherein and then drying to remove the solvent, so as to leave the dry,nontacky, relatively hard, uniform insulator film containing suspendedphotoconductor.

The silicone film forming materials heretofore employed are extremelysensitive to atmospheric conditions such as relative humidity so thatrecording elements produced from apparently identical solutions or evenfrom the same solution donot produce identical results. This hasnecessitated the use of careful control techniques and testingprocedures in attempts to insurereproducible results. With recordingelements produced using the composition of our invention, these controlprocedures may be substantially eliminated due to their relativeinsensitivity to changes in atmospheric conditions.

As stated above, the insulating films of the recording element arepreferably silicone resins, i.e. polysiloxanes. These resins as used inthe film are partially polymerized and must be capable of furtherreaction under the infiuence of the developer catalyst to form highmolecular Weight solvent resistant polymeric images. For convenience, Ishall refer to the relatively low molecular weight polymers of theinsulating film as the initial polymer and to the relatively highmolecular weight polymer of the polymeric image as the final product.

It is essential that the initial polymer contain functional groupscapable of further reaction to produce the final polymer. This has beenachieved in the past by utilizing initial polymers with a high hydroxylgroup content. During curing, the hydroxyl groups condense to split outwater and form oxygen linkages which join the initial polymer moleculestogether eiher by chain lengthening or cross-linking or both to form thefinal polymer. A typical initial polymer widely employed in theproduction of recording elements is the methyl phenyl polysiloxanemixture available from General Electric Corporation as SR-82. This resinhas a high hydroxyl content and is, therefore, capable of undergoingfurther polymerization to form the final polymer. Other similar initialpolymers have been employed.

I'have discovered that if instead of using hydroxylated siliconeresinsas the initial polymer a mixture of hydroxylated silicone resins andalkoxylated siloxanes or silanes is employed, the recording elementwhich is produced is not subject to the disadvantages referred to above.Thus, for example, if the initial polymer comprises a mixture of ahydroxylated siloxane and an alkoxylated siloxane or silane such thatthe alkoxy content of the mixture on a dry basis is from about.0.3% toabout 2% by weight of the total resin, recording elements produced canbe used in the preparation of photoengraving plates or resists withsharply defined image boundaries. Moreover, the insulating film on therecording element is remarkably tolerant to changes in atmosphericconditions and the results are reproducible.

-I prefer to use lower alkoxylated aryl type silanes and siloxanes suchas diphenyl diethoxy silane, or ethoxylated phenyl siloxanes asadditives to the initial polymer. These silanes are prepared byesterifying diphenyl ,dichlorosilane.-The siloxanes are prepared byesterifying phenyl chlorosilanes followed by hydrolysis of the resultingproducts. Ethyl alcohol may be used as an esterifying agent but alcoholscontaining up to four carbon atoms may also be used in the preparationof silanes and siloxanes which are useful in our invention. The productsare commercially available from Union Carbide Corporation and others.Diphenyl diethoxy silane is a monomer with a molecular weight of 272.The ethoxylated phenyl siloxanes are polymers with approximate molecularweights of from 500 to 5,000. The alkoxy content, i.e. the averagenumber of alkoxy groups per molecule depends upon themanner ofpreparation and may vary from 5% by weight or lower to 36% by weight oreven higher. It is most convenient to use additives in which the alkoxycontent is within this range since these are commercially available, butit is not necessary to do so. Additives in which the alkoxy content isoutside of this range may be utilized and the proportion used adjustedso that the alkoxy content of the insulating film varies within thepreferred range and from 0.3% to 2% by weight on a dry basis.

The basic concept of my invention is the preparation of recordingelements with a photoconductive layer deposited thereon in which theinitial polymer of the insulating film comprises a mixture containing ahydroxylated silicone resin and an alkoxy silane or alkoxylated siloxanein which the alkoxy groups contain up to four carbon atoms. Theproportions of the ingredients are preferably selected so that thealkoxy content of the insulating film is from about 0.3% to about 2% byweight of the total resin on a dry basis.

The procedures employed utilizing the novel compositions of my inventionare the same as with previously known compositions. The metal base plateis cleaned and the surface which is to receive the insulating film iscoated with a binder composition such as a phosphate binder or achromate binder. The binder coating is similar to the coating usedwhenever a metal is to be coated with plastic as for example, when ametal surface is to be painted. Phosphate binder coatings are obtainedby treatment of the surface with phosphoric acid. Chromate coatings areformed by treatment of the surface with a mixture of sulfuric acid and asoluble chromate salt such as sodium dichromate. Binder coatings are notessential to this invention but aid in holding the insulating film tothe base.

The insulating film is formed by coating the metal base with acomposition comprising a photoconductor such as photoconductive zincoxide suspended in a solution of the initial polymer mixture. Usuallythe surface is coated by spraying. The solvent is then removed byevaporation either at room temperature or by heating. The insulatingfilm thus formed is preferably from about 0.4 to 0.8 mil inthicknessoSomewhat thinner films will produce useful results but theymay not pick up suflicient polymerization catalyst. Film more than 0.8mil in thickness may be used in some applications, but are generally notdesired since the image boundaries of the finished products tend tobecome less sharp due to diffusionof the polymerization catalyst.

The recording element is used as described above. The electrostaticimage is produced and the polymerization catalyst is deposited on theimage by immersing the recording element in a developer bath with thecatalyst suspended therein and preferably rinsed to remove extraneouscatalyst. Therecording element is then cured in a curving oven to effectfurther polymerization of the insulating film in the image area. Ifaluminum octoates are used as the catalyst, the preferred curingtemperature is from about 450 F. to 530 F. and the curing time is fromabout three to six minutes. The plate with the polymeric image formedthereon is then cooled either by standing at room temperature orquenched in a cooling liquid such as water, and the background removedby washing the plate with a solvent. The plate is then descummed orcleaned by the usual procedures and it is ready for etching.

A further unexpected advantage of the improved recording elements of ourinvention is that when rinsing is employed to remove extraneouscatalyst, less expensive rinse liquids are necessary. In the past, therinse liquids utilized have included hydrocarbon solvents mixed withsilicone fluids. I have found that in accordance with my invention,excellent results can be achieved with rinse liquids which do notcontain any silicone fluids.

While my invention has been specifically described as applied tophotoengraving, it can also be used in other application where arecording element is formed by depositing a photoconductive layer on ametal base for the purpose of producing a polymeric image. It can beused, for example in the production of printed copper electricalcircuits. For the production of electrical circuits, the recordingelement comprises a hotoconductive layer which is deposited on thecopper surface of a copper laminate and an electrostatic imagecorresponding to the desired circuit formed thereon. The polymer imageis formed and the background area dissolved as described above to leavethe bare copper. This copper is then removed by etching, for examplewith ferric chloride or ammonium persulfate solution. The polymer imagewhich covers the circuit is then removed with a solvent such as toluene,to leave bare copper in the desired configuration for the electricalcircuit.

Example I A zinc photoengraving plate is cleaned, pumiced and a bindercoating is applied. The treated metal is then coated to a thickness of0.6 mil (dry) with a mixture containing the following ingredients:

SR-82 60% solution in xylene ml 360 R830 60% solution in xylene ml 70Toluene ml 200 Xylene ml 600 Photoconductive zinc oxide g 520 Uraninedye (2% solution in methanol) ml 6 SR-82 is a silicone resin with a highhydroxyl content available from General Electric Company, Pittsfield,Mass.

R830 is an ethoxylated phenyl siloxane with an ethoxy content of 5% andis available from Union Carbide Corp., New York, NY.

The coated plate is dried below 180 F. with air circulation to producean insulating film with an ethoxy content of 1.4% by weight. The plateis charged, exposed and developed by dipping the plate in an aluminumoctoate suspension in a mixture of isooctane, heptane and siliconefluid. The liquid developer is evaporated from the plate and the plateis heated for about four minutes at 500 F. to effect polymerization ofthe image area of the film. It is then cooled by quenching in water andwashed with a 1:1 mixture of xylene and heptane to selectively removethe background portions.

The plate with the polymeric image thereon is rinsed with hot water,blown dry and etched to produce a photoengraving plate.

Similar plates are prepared with other resin mixtures containinghydroxylated silicone resins and sufiicient amounts of alkoxylatedsilanes or alkoxylated siloxanes in which the alkoxy group contains upto four carbon atoms in portions so that the alkoxy content of theinsulated film is from about 0.3% to about 2% on a dry basis. Thefurther polymerization of the image area is catalyzed by aluminumoctoate, resinate or other polymerization catalysts.

When recording elements prepared using these mixtures are compared withordinary recording elements they are found to be much less sensitivetocharges in atmospheric conditions. The polymeric images produced fromthese recording elements have sharp, well defined boundaries comparedwith those produced from ordinary recording elements.

What is claimed is:

1. In a recording element comprising a hotoconductive layer deposited ona metal base, said layer comprising photoconductive zinc oxide suspendedin an insulating film formed from a hydroxylated silicone resin, theimprovement which comprises the presence in said silicone resin film ofa substance selected from the group consisting of alkoxy phenyl silanesand alkoxy phenyl siloxanes, the alkoxy group of said silane andsiloxane containing up to four carbon atoms in such amount that thealkoxy content of the film is from about 0.3% to about 2% by weight.

2. A recording element as in claim 1 in which the alkoxy groups areethoxy groups.

3. A recording element as in claim 1 in which the metallic base is zinc,magnesium or copper.

4. In a process of producing a recording element including the step offorming a hotoconductive layer on a metal base, said layer comprisinghotoconductive Zinc oxide suspended in an insulating film formed from ahydroxylated silicone resin, the improvement which comprises thepresence in said silicone resin of a substance selected from the groupconsisting of alkoxy phenyl silanes and alkoxy phenyl siloxanes, thealkoxy group of said silane and siloxane containing up to four carbonatoms in such amount that the alkoxy content of the film is from about0.3% to about 2% by weight.

5. A process as in claim 4 in which the alkoxy groups are ethoxy groups.

6. A process as in claim 4 in which the metallic base is zinc, magnesiumor copper.

7. In a process of producing a polymeric image including the steps offorming an electrostatic image on a recording element, said elementcomprising a photoconductive layer deposited on a metal base, said layercomprising photoconductive zinc oxide suspended in an insulating filmformed from a hydroxylated silicone resin, developing said polymericimage by depositing a polymerization catalyst on said electrostaticimage and heating to effect further polymerization of said film, theimprovement which comprises the presence in said silicone resin film ofa substance selected from the group consisting of alkoxy phenyl silanesand alkoxy phenyl siloxanes, the alkoxy group of said silane andsiloxane containing up to four carbon atoms in such amount that thealkoxy content of the fihn is from about 0.3% to about 2% by weight.

References Cited UNITED STATES PATENTS 3,065,194 11/1962 Nitzsche et al.260-465 X 3,070,559 12/1962 Nitzsche et al. 26046.5 X 3,121,007 2/1964Middleton et al. 961 3,155,504 11/1964 Darnm et al. 961.8 3,231,3741/1966 Sciarnbi 961 NORMAN o. TORCHIN, Primary Examiner.

I. TRAVIS BROWN, Examiner.

C. E. VANHORN, Assistant Examiner.

1. IN A RECORDING ELEMENT COMPRISING A PHOTOCONDUCTIVE LAYER DEPOSITEDON A METAL BASE, SAID LAYER COMPRISING PHOTOCONDUCTIVE ZINC OXIDESUSPENDED IN AN INSULATING FILM FORMED FROM A HYDROXYLATED SILICONERESIN, THE IMPROVEMENT WHICH COMPRISES THE PRESENCE IN SAID SILICONERESIN FILM OF A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF ALKOXYPHENYL SILANCES AND ALKOXY PHENYL SILOXANES, THE ALKOXY GROUP OF SAIDSILANE AND SILOXANE CONTAINING UP TO FOUR CARBON ATOMS IN SUCH AMOUNTTHAT THE ALKOXY CONTENT OF THE FILM IS FROM ABOUT 0.3% TO ABOUT 2% BYWEIGHT.